Field of the Invention
The present invention relates generally to security alarm systems, and more particularly to an improved motion sensor for detecting movement of a vehicle or other moveable object which is to be protected, with the motion sensor providing an output signal which is used to initiate an alarm whenever a significant level of movement is detected by the sensor.
Over the recent past, the rate of theft of motor vehicles has increased significantly. This is due at least in part to the simple fact that since the price of motor vehicles has increased at a significantly greater rate than the rate of inflation, the practice of stealing motor vehicles has become progressively more and more lucrative. This unfortunate occurrence has driven consumers in ever-increasing numbers to either purchase vehicles with an anti-theft system installed, or, more commonly, to have such a system installed after the purchase of a motor vehicle.
The competition between manufacturers of such anti-theft systems has led both to improvements in the design of such systems, and to a substantial drop in the cost of such systems. The most common anti-theft systems in use operate in a substantially similar manner to detect an unauthorized intrusion into the vehicle in which the anti-theft system is installed, and to sound an alarm to indicate that an attempted theft is in progress. Such alarm systems typically utilize audible alarms generated either by repetitively actuating the vehicle's horn, or by actuating a siren located in the vehicle. Many alarm-systems also flash the vehicle's lights to visually indicate that the vehicle is being subjected to an attempted theft. A few alarm systems transmit a signal to a remotely located receiver, which generates a signal to inform the owner that the vehicle has been disturbed.
Virtually all of these alarm systems operate in a substantially similar manner in order to detect an attempted theft or break-in of the vehicle. The alarm systems use switches which detect whenever a door of the vehicle (and, in many cases, whenever the hood or the trunk of the vehicle) is opened. Pin switches are typically utilized to detect entry via a door (or the opening of the hood or the trunk). Unfortunately, many thieves gain entry by breaking a window rather than by opening a locked door.
Because of this possibility, most alarm systems also include a motion detector which will initiate the alarm whenever the vehicle is moved significantly. Such motion detectors typically utilize a sensor mounted in a fixed location in the vehicle, and detect when the vehicle is subjected to a significant degree of jarring or jostling. If an intruder gets into the vehicle, the vehicle will generally be disturbed sufficiently for the motion detector to detect the disturbance and initiate an alarm. Similarly, if the vehicle is bumped or moved, the motion detector will again detect the disturbance and initiate an alarm.
Such motion detectors for the most part have been piezoelectric transducers, which have a mass suspended from an arm used to generate a bending movement of the arm when the mass is subjected to acceleration in a particular axis. A piezoelectric material is typically located on the arm, and when subjected to strain due to a bending of the arm, the piezoelectric material will generate an electrical signal. Piezoelectric transducers of this type are inexpensive, rugged, and easy to work with in the context of an alarm system. Unfortunately, since they are generally subject to movement only in a single axis more than one piezoelectric transducer is generally necessary in order to make a motion detector which will function satisfactorily for use as a motion sensor to trigger a car alarm.
As might be expected, the art has generated at least one clever alternative to the use of piezoelectric transducers as the heart of the motion detector. U.S. Pat. No. 4,584,569, to Lopez et al., reexamined as Reexamination Certificate B1 U.S. Pat. No. 4,584,569, utilizes a moveable resiliently suspended magnetic member which is coupled to a pickup coil. The magnetic member has a small but appreciable mass, and is suspended by a resilient spring member arrangement such that movement of the magnetic member causes it to move in its proximity to the pickup coil. The pickup coil is within the field of the magnetic member such that a voltage is induced in the pickup coil due to the movement of the magnetic member.
The value of the mass of the magnetic member and the relative stiffness of the resilient member are selected by Lopez et al. such that the magnetic member will be moved in significant degree in response to even very small values of motion. Movement of the magnetic member toward and away from the pickup coil, from side to side relative to the pickup coil in any direction in a plane parallel to the winding of the pickup coil, or rotationally about an axis lying in a plane parallel to the winding of the pickup coil will be detected. The suspension element is a resilient member which lies, when relaxed, in a plane parallel to the plane of the winding of the pickup coil, and substantially in the plane containing the center of gravity of the magnetic member and its mounting structure.
Signals induced in the pickup coil are amplified, and are then supplied to a comparator in order to determine whether or not the detected movement is of a sufficient level to cause an alarm to be generated. With the exception of the sensor arrangement described above, the Lopez et al. device is essentially of a conventional arrangement differing little from many other systems which generate an alarm based upon a signal from a motion detector. The Lopez et al. sensor does, however, represent a substantial improvement in the art in that it is a single sensor which is truly sensitive to motion in all directions.
Unfortunately, the Lopez et al. motion sensor is not without its disadvantages. The Lopez et al. motion sensor will work well as long as it is not subject to an appreciable magnetic field. However, if an appreciable magnetic field is generated, even for a relatively short period of time, in the viscinity of the Lopez et al. sensor, the pickup coil may have a signal generated therein which will set off the alarm. This is particularly true since due to its inherent design the Lopez et al. pickup coil must be able to detect and pick up even very small magnetic signals. Accordingly, such magnetic fields can easily generate false alarms with the Lopez et al. sensor, and, as any vehicle alarm owner knows, nothing (other than having the vehicle stolen) is more frustrating than repetitive false alarms.
It is accordingly the primary objective of the present invention that it provide a motion sensor for use with an alarm system which is responsive to movement in all directions, but which is completely immune to the presence of magnetic fields. As such, it is an objective of the motion sensor of the present invention that, when installed in a motor vehicle, it be sufficiently sensitive to motions which are indicative of a security violation of the vehicle. It is a further objective that the motion sensor of the present invention be adaptable for use in other applications than vehicle security, being usable in any application requiring a sensor for detecting movement of the item in which the sensor is located.
It is also an objective that the motion sensor of the present invention require very little power to operate, since one of its primary applications is in a motor vehicle where it will be powered by the vehicle battery, possibly for an extended period of time. It is an additional objective that the motion sensor of the present invention require electronic components which are of small size, thereby allowing the overall sensor to be relatively compact to facilitate its installation into spaces of limited size. It is a further objective of the present invention that the motion sensor design take advantage of existing, well-developed technology, thereby enabling the production of a superior motion sensor at an attractive price.
The motion sensor of the present invention must also be of construction which is both durable and long lasting, and it should also require little or no maintenance to be provided by the user throughout its operating lifetime. In order to enhance the market appeal of the motion sensor of the present invention, it should also be of inexpensive construction to thereby afford it the broadest possible market. Finally, it is also an objective that all of the aforesaid advantages and objectives of the motion sensor of the present invention be achieved without incurring any substantial relative disadvantage.